Silicon-based photonic components working at 1330 and 1500 nm fiber-optic communication wavelengths for various systems such as fiber-to-the-home and local area networks (LANs) are a subject of intensive research as a result of the possibility of integrating optical elements and advanced electronics together on a silicon substrate using the well-known techniques of CMOS technology.
Passive silicon structures, such as waveguides, couplers and filters have been extensively studied. Less work has been reported on active silicon devices (i.e., tunable devices), such as modulators and switches, despite their importance as a means of manipulating light beams for such communication systems. Some silicon-based thermo-optic active devices have been designed, where the refractive index of the silicon is modulated by varying the silicon temperature, thereby inducing a phase modulation and absorption which in turn is used to produce an intensity modulation at the output of the device. Nevertheless, the thermo-optic effect is rather slow and can only be used for device speeds up to 1 Mb/s modulation frequency. Therefore, for higher modulation frequencies (which are of more interest in most systems, including communication systems), electro-optic active devices are required.
Most of the proposed electro-optic devices exploit the free carrier dispersion effect to change both the real and imaginary parts of the refractive index. This exploitation is used since the unstrained pure crystalline silicon does not exhibit a linear electro-optic (Pockels) effect, and the refractive index changes due to the Franz-Keldysh effect and Kerr effect are very weak. In free carrier absorption modulators, as will be discussed in detail below, changes in the optical absorption of the structures are directly transformed into an output intensity modulation. Phase modulation in a specific region of optical devices, such as Mach-Zehnder modulators, total-internal-reflection (TIR)-based structures, cross switches, Y-switches, ring resonators and Fabry-Perot resonators, is also used to modulate the output intensity.
Free carrier concentration in electro-optic devices can be varied by injection, accumulation, depletion or inversion of carriers. Most of such devices investigated to date present some common features: they require long interaction lengths (for example, 5-10 mm) and injection current densities higher than 1 kA/cm3 in order to obtain a significant modulation depth. Long interaction lengths are undesirable in order to achieve high levels of integration and miniaturization for fabricating low-cost compact device arrangements. High current densities may induce unwanted thermo-optic effects as a result of heating the structure and will, indeed, cause an opposite effect on the real refractive index change relative to that associated with free carrier movement, thus reducing its effectiveness.
FIG. 1 illustrates an exemplary prior art, silicon-based electro-optic phase modulator 1 formed using a raised rib waveguide on an SOI structure. Electro-optic phase modulator 1 includes a layer of intrinsic (single crystal) silicon 2 that has been processed to include a rib structure 3 that extends transversely (as shown in the insert) to form the optical waveguide of modulator 1, where the direction of optical signal propagation is also shown in the insert. Intrinsic silicon layer 2 is illustrated as the top layer of a conventional silicon-on-insulator (SOI) wafer structure, which further comprises a buried oxide (BOX) layer 4 and silicon substrate 5. The structure as shown forms a PIN diode modulator and is arranged to vary the refractive index in silicon rib waveguide 3 by using the free carrier dispersion effect, as mentioned above. In this particular example, silicon layer 2 is formed to include a heavily-doped p-type region 6 in contact with a first electrical contact 7. Layer 2 further includes, as shown, a heavily-doped n-type region 8 and associated second electrical contact 9. In one example, regions 6 and 8 may be doped to exhibit a dopant concentration on the order of 1020 carriers per cm3. In this PIN structure, p-type region 6 and n-type region 8 are spaced apart on opposite sides of rib 3 so that intrinsic silicon lies between the heavily doped regions both in rib 3 and silicon layer 2.
In operation, first and second electrical contacts are connected to a voltage supply so as to forward bias the diode and thereby inject free carriers into waveguide 3. The increase in free carriers changes the refractive index of the silicon (as discussed using the Drude model, below) and can therefore be used to achieve phase modulation of light transmitted through the waveguide. However, to act as an optical modulator, the speed of operation of electro-optic modulator 1 is limited by the lifetime of free carriers in rib 3, as well as the carrier diffusion rates when the forward bias is removed. Such prior art PIN diode phase modulators typically have a speed of operation in the range of 10-50 Mb/s for forward biased operation. By introducing impurities into the silicon, which act as carrier lifetime “killers”, the switching speed can be increased, but the introduced impurities detrimentally affect the optical transmission. However, the primary impact on speed is due to the RC time constant product, where the capacitance (C) in forward bias becomes very large due to the reduction in the depletion layer width of the PN junction in forward bias. Theoretically, high speed operation of a PN junction could be achieved in reverse bias, although this would require large drive voltages and long device lengths, which are incompatible with the CMOS process.
There remains, therefore, an urgent need for optical modulator structures based on the electro-optic effect that can be implemented in a sub-micron region while offering low cost, low current density, low power consumption, high modulation depth, low voltage requirements and high speed modulation.